1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a control method for a liquid ejection apparatus, and more particularly, to a liquid ejection apparatus and a control method for a liquid ejection apparatus in which vibrating action is performed to free surfaces of liquid in nozzles.
2. Description of the Related Art
A liquid ejection apparatus is known which performs vibrating action to liquid in order to prevent the viscosity of the liquid from increasing nearby a free surface (the liquid-atmosphere interface, which is also commonly called “meniscus”) of the liquid inside a nozzle (a liquid ejection port).
More specifically, as shown in FIG. 12A, the solvent of ink evaporates from the free surface of the ink in a nozzle 51 due to the difference in the vapor pressure of the solvent between the atmosphere and the ink in the nozzle 51, and as shown in FIG. 12B, the ink inside the nozzle 51 increases in viscosity and solidifies, then the nozzle 51 becomes clogged, thereby creating a state in which ejection is not possible. In order to prevent the nozzle 51 from being clogged, when the nozzle 51 is at rest and ejection is not performed, vibrating action is repeatedly performed for the ink to make the free surface of the ink in the nozzle 51 vibrate slightly to an extent that does not cause ejection of the ink (this action is commonly called “meniscus vibrating action”, and is hereinafter referred also to simply as the “vibrating action”).
By performing the vibrating action, the increase in the viscosity of the ink in the vicinity of the free surface is restricted due to the churning of the ink inside the nozzle 51, and moreover, as shown in FIG. 12A, the ink inside the ejection flow channel 512 leading to the nozzle 51 and the ink inside a pressure chamber 52 installed with an actuator 58, is also churned. Moreover, the ink inside a supply flow channel 953 which supplies the ink to the pressure chamber 52, and inside a portion of a common flow channel 955, is also churned.
Japanese Patent Application Publication No. 2001-270134 discloses a liquid ejection apparatus which performs the vibrating action on the basis of prescribed churning conditions, at a position where the apparatus has been returned to a standby position by passing through a prescribed distance from the liquid droplet ejection starting point, wherein at least one of the ambient temperature and the ambient humidity is measured, the number of pulses which constitute an electrical signal for the vibrating action (vibration pulses) can be increased or decreased (in other words, the vibrating action duration can be increased or decreased), the frequency of the electrical signal for the vibrating action can be changed, and the amplitude of the electrical signal for the vibrating action can also be changed.
Japanese Patent Application Publication No. 9-290505 discloses a liquid ejection apparatus which performs the vibrating action for a nozzle that has not performed ejection for a specific time period or more, wherein the specific time period is approximately one half of the time period required until the nozzle becomes clogged when the surface of the liquid in the nozzle is exposed to the atmosphere.
However, if the vibrating action to the free surface of the ink is performed repeatedly during an idle duration in ejection, then although the viscosity of the ink in the vicinity of the free surface is reduced for a short while, the solvent evaporates again from the free surface of the ink in the nozzle 51 where the viscosity has been reduced once, and the ink viscosity continuously rises in the whole of the flow channel, including the ejection flow channel 512 to the nozzle 51 and the pressure chamber 52. Therefore, eventually, beneficial effects cease to be obtained, even if the vibrating action is performed. Consequently, if the vibrating action is repeatedly performed, then it eventually becomes necessary to perform dummy ejection, also known as purging, before reaching a state where ejection has become impossible.
If the ink viscosity is to be restored by the purging, then it is necessary to discard the ink inside the pressure chambers 52 in a state where the ink in the whole flow channel, including the pressure chamber 52, has increased in viscosity due to the vibrating action as described above.
In FIG. 12C, the first line 911 indicates the initial solvent concentration distribution (in other words, the distribution where the average value of the solvent concentration of the ink from the common flow channel 955 to the nozzle 51 is the initial concentration A), the second line 912 indicates the solvent concentration distribution after a prescribed time period t1 has elapsed, and the third line 913 indicates the solvent concentration distribution after a time period t2 (>t1) has elapsed. As shown by the lines 911, 912 and 913 indicating these solvent concentration distributions, with the passage of time, the solvent concentration of the ink in the pressure chamber 52 falls more slowly (in other words, the ink viscosity rises more slowly) than in the nozzle 51; however, if the ink of unsatisfactory solvent concentration is to be discarded by purging in order that normal ejection can be achieved, then not only the ink inside the nozzle 51, but also the ink inside the pressure chamber 52 must inevitably be discarded. In other words, it is necessary to discard an amount of ink corresponding to an obliquely shaded region 902 which indicates the purging volume in FIG. 12C.
In particular, if it seeks to arrange a plurality of nozzles 51 at high density, then the volume of the pressure chambers 52 becomes smaller. In this state, if the vibrating action is continued, then the ink inside the pressure chamber 52 rapidly increases in viscosity. Therefore, the purging interval becomes shorter. Furthermore, ink of a volume that is directly proportional to the number of nozzles must be discarded.
Here, the issue of the purging process in a liquid ejection apparatus in the related art is described in more detail with reference to FIG. 13. In FIG. 13, the horizontal axis denotes time t, and the vertical axis denotes the solvent concentration of the ink at a position 920 in the nozzle 51 shown in FIG. 12A, which position is distant from the free surface of the ink and in the proximity of the pressure chamber 52. The first line 921 shows the temporal change of the solvent concentration of the ink at the position 920 in a case where the vibrating action is performed and purging is not performed, the second line 922 shows the temporal change of the solvent concentration of the ink at the position 920 in a case where the vibrating action and the purging are performed, and the third line 923 shows the temporal change of the solvent concentration of the ink which can be restored by the purging. If the vibrating action is performed repeatedly from the solvent concentration A in the initial state (t=0) immediately after ejection, then as stated above, the solvent concentration of the ink falls (P0 to P1), and therefore, immediately before the solvent concentration reaches the limit solvent concentration E at which normal ejection is still possible, for example, when the solvent concentration has fallen to a threshold value thE in the vicinity of the limit solvent concentration E, the purging is performed. When the purging is performed, the solvent concentration of the ink is restored (P1 to P2). Subsequently, the vibrating action and the purging are repeated continuously (P2 through P9). Since it is necessary to perform the purging repeatedly in this way, then an amount of ink corresponding to the purging volume 902 shown in FIG. 12C is repeatedly discarded from each nozzle 51. Furthermore, since there is a gradual decline in the level to which the solvent concentration of the ink can be restored by the purging, as indicated by the third line 923, then it is also necessary to re-initialize the solvent concentration of the ink by performing a more fundamental maintenance operation, such as suctioning (P9 to P110). During purging or during a maintenance operation, it is not possible to perform ejection for its original purpose. In other words, a waiting time arises during which it is not possible to perform printing. Furthermore, suctioning generally requires longer time than purging.